RICE. And i don't mean the white kind in a bowl with soy sauce. Rest-ice-compression-elevation, especially if it's tender, hard to bend, and hurts at the end of the day. This can help relieve the swelling and pain, along with nsaid's. Keep the knee active, limit high impact activity on it though.

See below. First step is to reduce inflammation. This includes rest, antiinflammatory meds, ice. When inflammation is decreased, gentle range of motion exercises begin. As painfree range of motion returns, the knee muscles can be strengthened and then exercises can be advanced to sports specific exercises if this is an athlete.